In recent years technological innovations, such as increases in the amount of iron discharged because of the larger size of blast furnaces and the operation the furnaces under high pressure, significant decreases in coke ratios due to dust coal and the like being blown in and significantly extended amounts of operating time of blast furnaces, have produced harsh service conditions for a mud material, increasing the demand for higher quality product. In addition, the inherent properties of the mud material demanded have changed since the quality of the environment must be considered.
The mud material needs to have properties for protecting bricks around the iron tap hole on the furnace wall and for improving the working atmosphere, in addition to the conventional properties of corrosion resistance and workability in order to block the iron tap hole and to discharge slag and hot metal.
In various types of existing blast furnaces, there are from one to four iron tap holes in each furnace. Because these iron tap holes are alternately used in the ordinary operation of discharging hot metal, the same iron tap hole is opened and blocked usually at intervals of several hours. In one operating method, only two diagonal iron tap holes are alternately used over one week, and then an iron tap hole out of operation is used. There are times when one iron tap hole is used continuously because of the status of a runner, trouble with a mud gun or the like. In such a case, there are times when the iron tap hole must be blocked or opened for approximately one hour.
From the viewpoint of versatile operation, the properties required for the mud material are as follows:
1 The mud material should have an optimum viscosity and plasticity so that the iron tap hole can be easily filled with the mud material by a mud gun; PA1 2 The shorter amount of time required to calcine the mud material fed inside the furnace, the better. There should only be a small amount of smoke when the iron tap hole is opened; PA1 3 The sintering strength of the mud material after the material has been calcined should not depend significantly on the amount of calcining time. Opening the iron tap hole should be easy; PA1 4 The mud material should have corrosion resistance with respect to slag and hot metal so that it can be deposited in the furnace for a long period of time. The bore of the iron tap hole should only increase a small amount, and the speed at which iron is discharged can be maintained stably; PA1 5 The mud material should have excellent adhesion to old material at high temperatures when it is fed into the furnace. It should have excellent high-earty strength and properties for stabilizing the volume when it is heated rapidly. The mud material should be capable of protecting the wall bricks of the furnace and stably forming and maintaining the depth of the iron tap hole; PA1 6 The mud material should not contaminate the working environment.
Grain size and various refractories and binders have been conventionally examined so that mud materials used for an iron tap hole in a blast furnace are provided with the above-mentioned properties. For instance, alumina type material whose main components are high-alumina, electro-molded (or sintered) alumina, silicone carbide, silicone nitride, coke and clay are mainly used in place of a silicate type and low alumina type materials whose main components are chamotte, Roseki, coke and clay. In order to improve the working environment there are presently plans to switch from a coal type tar mainly used as a binder to a petroleum type or a resin type solution.